Lecture 2—Adsorption at Surfaces

1.Adsorption/Desorption

2.Overlayers, lifting reconstruction

3.Dissociative and Associative adsorption

4.1st and 2nd order desorption, kinetics

Reading—Lecture on Langmuir adosrption isotherm

P. A. Redhead, Vacuum 12 (1962) 203-211

Smentkowski and Yates, Surf. Sci. 232 (1990) 1

S S S SS SAAA

A AAdsorption at open site: S = S0

No adsorption at occupied site: S = 0

Monolayer Adsorption:

Assume an Adsorbate (A) can adsorb at an empty surface site with sticking coefficient S0.

If a surface site is already occupied, then no adsorption occurs (S = 0)

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(1x1) unit cell

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Adsorbate forming a (2x2) overlayer on the (1x1) substrate surface

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Unused surface bonds can interact, causing change in surface structure

Surface dimerization

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Surface dimerization

Dissociative adsorption of H2

HHHHHH

Adsorption can induce or lift surface reconstructions (e.g., H/Si(100)

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Associative Adsorption adsorbate does not break bonds during chemisorption, e.g., CO/W

W W W W W W

CO

CO

CO

CO

CO

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W W W W W W

H H

H2

Dissociative adsorption: molecular bonds broken during adsorption

3.g., H2 on W

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Consider adsorption (assoc. or dissoc.) at a surface

In this process, note that the average sticking coefficient will depend on the fractional surface coverage (Θ).

At zero or low coverage, the sticking coefficient will be S0: 0<S0<1

At Θ = 1 (full coverage), S = 0

For 0<Θ<1, we have S = S0(1-Θ)9

We can therefore model adsorption as an equilibrium between adsorbate, and surface sites:

[A] + [S] [AS]

Therefore:

(1)Kads = [AS]/[A][S]

We can then rewrite [AS] and [S] in terms of Θ:

(2)[AS]/[S] = Θ/(1-Θ)

Given that [A] is some constant (C), we can re-write (1) as

Kads = Θ/C(1-Θ), or

(3) Θ = KadsC/(1+KadsC)(the usual form for the Langmuir Isotherm)

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(3) Allows us to determine the fractional surface coverage as a function of C (typically proportional to pressure)

For small C, KadsC << 1, and

Θ ~ KadsC

For large C, we have KadsC >> 1

And Θ ~ 1

Θ

C

Θ~ KadsC

Θ~1

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In many laboratory situations, adsorption at a given temperature is, for all practical purposes, irreversible.

Kads is very large, and the equilibrium fractional surface coverage is 1.

However, we are often concerned with the kinetics of coverage, as we can control the total exposure of a gas to the surface. Assuming that once adsorbed at a given temperature, A will not desorb, we have

Θ = SFt where S = sticking coefficient, F = flux to the surface (suitably normalized, and proportional to pressure), and t = time.

Ft = “exposure”. Typically, exposure is measured in “Langmuirs” (L) where L = 10-6 Torr-sec

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Note, however, the S is not constant.

We have S = S0(1-Θ)

We then have :

(4) Θ = S0(1-Θ)cPt where c is a constant, and P = Pressure

Differentiating both sides of (4) with respect to t,

(5) dΘ/dt = S0cP – S0cP(dΘ/dt)

For Θ << 1, we have

dΘ/dt ~ S0cP, and coverage will increase linearly with exposure

For Θ ~ 1,

(6) then dΘ/dt = 0, and coverage is constant with time 13

Θ

P x t

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Interrogating adsorption and desorption

Temperature Programmed Desorption (TPD)Temperature Programmed Reaction Spectroscopy (TPRS)

Typical experimental apparatus(Gates, et al., Surf. Sci. 159 (1985) 233

Typically need:

•QMS with line of site

•Controlled dosing

•Temperature control with linear ramp

•Another method to monitor surface composition/structure (Auger, XPS, LEED…) 15

TPDStep 1:Adsorption at Low Temp

Step 2:Desorption vs. temperature: dT/dt ~ 1-10 K/sec

QMS

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1st and 2nd order desorption

1st order, desorption occurs from a surface site

2nd order, desorption occurs after surface reaction and combination, e.g.;

Hads + Hads H2 desorbed

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In monitoring desorption from a surface, the desorption rate (N(t); molecules/cm2-sec) is proportional to two pressure-dependent terms (see Redhead)

aN(t) = dp/dt + p/λ

a = constant (dependent on surface area) p = pressure λ = pumping time (reciprocal of pumping rate)

In modern vacuum systems, λ is very small, and p/λ becomes the dominant term.

The desorption rate is therefore proportional to the (partial) pressure as measured by the QMS:

N(t) = kp

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We can therefore monitor the desorption rate by looking at the change in partial pressure of the desorbing species (e.g., H2, CO, etc.) in the QMS:

PCO

T ( = T0 + βt)

Temperature of desorption rate maximum

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We can (Redhead) express N(t) as the product of an Arrhenius rate equation:

(7) N(t) = -dѳ/dt = vnѳn exp(-E/RT)

v = rate constant n = order of the reaction ѳ = concentration of adsorbates (molecules/cm2) E = activation energy

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Given that T = T0 + βt (the linear heating rate is critical), we can solve (7) for the temperature at which N(t) is a maximum (T = TP) (Redhead, again)

For n = 1 (1st order) (v1 is the first order rate constant)

(8a)E/(RTp2) = ( v1/β)exp(-E/RTp) TP independent of surface coverage

(note: this assumes E does not vary with surface coverage; not always true)

For n = 2 (2nd order)

(8b) E/RTP2 = (σ0v2/β)exp(-R/TP) TP is a function of initial surface coverage (σ0)

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From 8a, b, we can see that one way to distinguish 1st from 2nd order desorption processes is to due the same desorption expt at different surface coverages:

Initial coverage, 0.7 ML

Initial coverage, 0.4 ML

Initial coverage, 0.4 ML

1st order reaction, temp. of peak maximum is invariant with initial coverage.

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2nd Order: Peak temperature decreases with increasing initial coverage (desorption of H2 from W: Redhead)

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